It's kind of like an online book and game at the same time. You create an account, and then you begin exploring the first few chapters of Philosophers Stone. You can read extracts from the book, and you also have to collect items on your way to move on to the next chapter. Once you've made it into Hogwarts and gotten into a house, you can begin having duels with other users, and you can make potions to earn extra house points. That's pretty much it. It's really simple to use.. But it gets boring after a while.
Pottermore is a free on-line website and experience that is based on the Harry Potter series of books, and the wizarding world.